How is your due date calculated?

Gemma 👼🏼👧🏼👦🏼

I’ve recently found out I’m pregnant and I’m confused as to how many weeks pregnant i am. So my last period was on the 28th Jan. if I take that date I am 4 weeks 2 days.

If I take my start date and then my average cycle of 32 days it goes to 3 weeks 4 days

Does anyone know how they calculate it?